Stainless steels are usually welded by using welding materials with a chemical composition fundamentally the same as that of the base stainless steel. In doing so, some problems regarding a depression of mechanical properties and corrosion resistance of the weld metal may arise due to types of the base metal. For example, in an operation of welding a martensitic stainless steel, containing 9 to 14% chromium (hereinafter "%" means "weight %" for any alloying component), using the welding materials with the same composition as that of the base metal, the weld metal is too hard and not able to exhibit the desired level of toughness, which causes sometimes inferior resistance to stress corrosion cracking, depending on the circumstances.
In order to solve these problems, some inventions were proposed and disclosed in Japanese Patent Public Disclosure No.8-57683 and International Patent Public Disclosure WO97/12072, each being characterized by welding base metals of martensitic stainless steel, with a welding material of duplex stainless steel that consists of austenite and ferrite.
On the other hand, some novel types of stainless steels, as the base metals, have been recently developed. However, any welding material suitable for use in welding these base metals has not yet been developed. Among these new types of stainless steels, there is one named "super duplex stainless steel".
Since the austenite-ferrite duplex stainless steel (JIS SUS329 and the like) exhibits high corrosion resistance, toughness and strength, it has been extensively used in structural materials which shall be exposed to the severe corrosive circumstance.
Early duplex stainless steels, in other words the first generations of duplex stainless steels, were defective regarding both workability and weldability. Then, the defects were overcome by adding nitrogen etc. Thus improved stainless steels might be called the second generations of duplex stainless steels, and they are applicable for wider uses. For example, in recent years, petroleum resources have become gradually exhausted, and most oil wells can be found at locations of severer conditions, wherein materials OCTG and line pipes are required to have more excellent corrosion resistance, higher toughness and the other mechanical properties. In order to meet such requirements, further new duplex stainless steels with higher content of nitrogen or other alloying elements have been developed. Those stainless steels are called "super duplex stainless steels", and are now drawing the attention.
These duplex stainless steels are generally welded with welding materials having the chemical composition similar to that of the base stainless steels. This means that the nitrogen content of the welding material is increased in accordance with the grade of the base stainless steels. In Japanese Patent Application No.7-60523 (Japanese Patent Public Disclosure 8-260101), the applicant of this application has already proposed a welding material for a duplex stainless steel, which contains 24 to 26% chromium, 2.0 to 3.3% molybdenum, 1.5 to 5% tungsten and a high level nitrogen of 0.24 to 0.35%. The welding material is a solid wire and made of a duplex stainless steel with the composition similar to that of the base steel. With using this welding wire, both the pitting corrosion resistance and toughness of the weld metal can be improved remarkably.
The welding wire with such a high nitrogen content is applicable to tungsten inert gas welding method (hereinafter designated as TIG), but not always applicable to consumable electrode welding method, e g., metal inert gas welding method (hereinafter designated as MIG), since welding defects called "blowholes" are apt to be produced. Particularly, the blowhole defects are often produced in the overhead position welding in comparison to other positions welding. Therefore, it is doubtful whether the welding wire made of a duplex stainless steel, with such high nitrogen content, is applicable to MIG in all positions welding.
Regarding the solid welding wire, having a composition similar to that of the base metal of the duplex stainless steel with high nitrogen content, there is another problem as follows.
The duplex stainless steel contains large amounts of chromium, molybdenum and other alloying elements, which are effective to increase corrosion resistance, such as resistance to pitting corrosion. These alloying elements accelerate precipitation of hard and brittle intermetallic compounds (.sigma. phase) in the matrix, and wire drawing operation of the duplex stainless steel becomes difficult. This means that a production of duplex stainless steel welding wire and also its stable supply to customers are difficult.
In addition to the above-mentioned MIG using a solid welding wire, there is another gas shield arc welding method, using consumable electrode, in which a flux-cored wire (composite wire) is used. Since the flux cored wire envelops slag forming agents other than alloying elements. It has various advantages such as little sputter generation and favorable bead formation.
However, in case of welding metals with conventional flux cored welding wire, cleanness of the weld metal is far inferior to that of the weld metal formed by the other welding method. The inferior cleanness has been a barrier to an improvement of the toughness and pitting corrosion resistance of the weld metal. The reason for the poor cleanness of the weld metal during the welding operation is based on the fact that a part of slag forming agents, most of which are oxides, remains in the weld metal as non-metallic inclusions.
Japanese Patent Publication No.8-25062 discloses a flux cored welding wire for use in welding a stainless steel, which forms little amounts of slag during welding operation. This flux cored welding wire, however, was developed so as to simplify a sequence of welding operations or eliminate any one of operation steps, e.g., a slag-removing step. Accordingly, this welding wire is not suitable for use in welding the duplex stainless steels, in particular, the super duplex stainless steels.
As particularly mentioned above, any consumable electrode welding material for use in welding the super duplex stainless steels, which is capable of forming sound weld metal in all welding positions, have not been developed yet. At present, only a solid wire, having the composition similar to that of the base metal, and being applicable to the TIG welding, is produced.
The invention, which is disclosed in the above-mentioned Japanese Patent Public Disclosure No.8-57683, relates to a method of welding martensitic stainless steel using a filler metal of duplex stainless steel. The composition of the filler metal to be used in this method is restricted in view of ensuring the grade of mechanical properties and corrosion resistances of the weld metal. The above-mentioned problems regarding manufacture of the welding wire, generation of blowholes during welding operation and the poor cleanness of the welding metal, have not been solved. The situation is the same for the invention of above-mentioned international patent public disclosure WO97/12072. Although it is a good idea to weld martensitic stainless steels using a welding material of the duplex stainless steel, the welding material has not yet been fully improved in order to substantiate the idea.